The development in lymphocytes of cytotoxic function against histocompatibility complexes is being studied as a problem in cellular differentiation. Two approaches are being taken. First, we are doing a complete analysis of macromolecular synthetic patterns, in order to detect changes in expression of genetic information during development of cytotoxicity. Along these lines we have also examined the effect in this system of Burk, whch under approximate conditions may selectively inhibit cell differentiation. Secondly, since the results of our first approach are beginning to suggest very strongly that development of cytotoxicity may not involve expression of neo genetic information, we are investigating other mechanisms that might allow such a functional cellular differentiation. We are especially interested in the development of recognition function by lymphocytes during sensitization, which appears concomitantly with cytotoxic function. Our thesis is that development of cytotoxicity may be the facilitation of a preexisting function, by promotion of binding of the (potentially) cytotoxic lymphocyte to the appropriate target. We will be investigating the mechanisms underlying recognition, and the control of its inception.